Plasma addressing structures may be employed in a variety of applications, including video cameras, memory devices, and flat panel displays, including flat panel liquid crystal displays. Such addressing structures and storage devices are described in U.S. Pat. No. 4,896,149 to Buzak et al. for "Addressing Structure Using Ionizable Gaseous Medium" and in U.S. Pat. No. 5,077,553 to Buzak for "Apparatus and Method of Addressing Data Storage Elements," both assigned to the assignee of the present application.
One embodiment of a conventional plasma addressing structure includes a display screen with first and second spaced-apart glass substrates positioned face to face with each other. A layer of dielectric material and a layer of, for example, liquid crystal material separate the first and second substrate faces. Multiple parallel electrical conductors extend generally in a first direction along the inner surface of the first substrate to form column data electrodes for receiving data drive signals. Multiple parallel channels inscribed into the inner surface of the second substrate extend along the inner surface in a second direction generally transverse to the first direction.
Each of the channels is filled with ionizable gas. The gas is ionized by a gas ionizing structure that includes an electrical reference electrode or anode and a cathode that extend along the length of each channel. The gas in each channel is selectively ionized to address elements aligned with the channel. The addressable elements may include a row of display elements having electro-optical properties, such as the layer of liquid crystal material positioned between the inner surface of the first substrate and the dielectric layer.
Sidewalls between adjacent channels define a plurality of support structures whose top surfaces (referred to as "land areas") support the layer of dielectric material. The layer of dielectric material functions as an isolating barrier between the ionizable gas contained within the channels and the addressable elements. The dielectric material prevents the addressable material from flowing into the channels and prevents the ionizable gas from contaminating the addressable material.
The first and second substrates and the dielectric layer are typically bonded together along their boundary regions by glass frit. The fritting process usually is carried out in an oven at elevated temperatures under ambient nitrogen or oxygen. Heating at elevated temperature in ambient oxygen or nitrogen helps drive water vapor from the memory device, display screen, or other plasma addressed system. Water and other volatile species are believed to shorten the commercially useful life of such plasma addressed systems.
To retain desired electrical and optical qualities, the dielectric layer is preferably extremely thin. The dielectric layer has been formed of various materials, including glass, mica, or thermoplastic. Each of these materials has significant limitations and has proven unsatisfactory for some purposes. Bonding such materials to a substrate has also presented serious challenges.
A sheet of very thin glass is conventionally used as the dielectric layer in plasma addressed display screens. One example is Schott.RTM. D263 glass sheets which are 0.05 mm (2 mils) in depth (thickness), optically clear, thermally stable, durable, and nearly chemically pure. Such a thin glass sheet has been thought to have the most satisfactory combination of electrical, physical, and optical qualities of the materials commonly used as the dielectric layer in such displays. However, mating the thin glass sheet with the first and second glass substrates has proven difficult.
A thin glass sheet is extremely brittle, fragile, and difficult to handle. Glass frit or other particulate contamination on either the thin glass sheet or a land area often results in fracturing the glass sheet during bonding with the substrate layers. Consequently, the utmost care must be taken to avoid particulate contamination. A thin sheet of mica presents these same problems and in addition has less satisfactory optical properties than a glass sheet.
Additionally, glass sheets and mica sheets are available from a limited number of producers and in a limited number of sizes. Such thin glass sheets and thin mica sheets are also very expensive. Large sheets of thin glass, such as would be required for large screen flat panel displays, are unavailable. Consequently, the unavailability of large-sized, dielectric sheet material having desirable properties has placed an upper limit on the dimensions of the display area of plasma addressed display screens.
Thermoplastics are typically extruded. Thermoplastics tend to develop cracks, to scratch easily, and are difficult to obtain in sheets which are sufficiently thin and uniform. It is believed desirable that the dielectric layer bond only to the land areas rather than encroaching upon the channel sidewalls. Because thermoplastics are generally malleable and melt at low temperatures, thermoplastic sheets tend to deform unacceptably into the channels rather than spanning from land area to land area and remaining relatively flat and rigid. Additionally, many thermoplastics are not thermally stable and undergo unacceptable discoloration during the high temperature fritting cycle. Therefore, thermoplastics have not been used as dielectric layers for plasma addressed systems.